On correcting Observations made with Equatorial Instruments. By 

 T. R. Robinson, B. D. M. R I. A. Professor of Astronomy at 

 Armagh. 



Read January 10, 1825. 



On being appointed to the Observatory at Armagh, the principal 

 instrument of which is a magnificent Equatorial by Troughton, I 

 was anxious to collect all possible information connected with the 

 mode of using it. But in this I had little success ; astronomers have 

 investigated the theory of the Transit and Vertical Circle thoroughly, 

 and almost every treatise on the science contains a variety of for- 

 mulae to facilitate their employment, while the essay of Sir G. 

 Shuckburgh, and a chapter of Professor Vince's Pract. Ast. were 

 my sole resources. My predecessors here seem to have felt the same 

 difficulty ; at least they have recorded no observations made off the 

 meridian: I am even informed that it was in contemplation to 

 mount the Equatorial in a vertical position, but this would certainly 

 be unjust to its admirable maker, who I think never displayed 

 greater ability than in its construction. At first sight nothing can 

 be more facinating than the apparent facility which an Equatorial 

 offers to the astronomer ; the determining a star's place by an im- 

 mediate reference to the pole instead of the zenith pleases by its sim- 

 plicity ; far more numerous observations can be taken of the same 

 star in each day, and the power of observing with accuracy off the 

 meridian, is of inestimable advantage in this variable climate. The 

 great service which a telescope, provided with a micrometer and 

 equatorial stand, can render astronomy is well known ; and it may 



*jj2 



